"Why, father," replied Rollo, "I thought that wind was motion of the air."
"So it is," said his father; "but all motion of the air is not wind. Wind is a current of air, that is, a progressive motion;—and in fact, there is, this morning, a slight current from the westward."
"How can you tell, father?" asked Rollo.
"By the smokes from the chimneys; don't you see that they all lean a little from the west towards the east?"
"Not but a little, father;—and there's one, from that red house, which goes up exactly straight."
"Yes," said his father, "there is one; but, in general, the columns of smoke lean; which is proof that there is a gentle current of air to the eastward."
"Westward, you said, father," rejoined Rollo.
"Yes, from the westward, but to the eastward.
"That is what is called a progressive motion," continued Rollo's father; "that is, the whole body of air makes progress; it advances from west to east. But there is another kind of motion, called a vibratory motion."
"What kind of a motion is that, father?" asked Rollo.